Where The Streets Have No Name is the greatest song EVER written IMO, and I mean EVER. It is the epic that defines U2 for many diehard fans, and I believe it is the song that defines U2 for casual listeners as well. It is the point from which all other songs must be measured, and so far, in my mind, nothing comes close.
Despite this, there are other U2 epics that have a very deep effect on me, that I consider to be masterpieces, such as: Bad, All I Want Is You, One, Pride, and New Year's Day (to name a few...)